Surface and Thin Films Analysis

13 Surface and Thin Films Analysis R. K. SKOGERBOE DEPARTMENT OF CHEMISTRY COLORADO STATE UNIVERSITY FORT COLLINS, COLORADO

13.1 13.2 13.3

13.1

Introduction General Statement of the Analysis Problem Instrumentation and Applications 13.3.A Spark-Source Mass Spectrometry 13.3.B Laser Methods 13.3.C Ion Bombardment Instruments References

401 402 404 404 408 410 421

INTRODUCTION

The physical and mechanical properties of many materials and/or devices may be dramatically affected by the presence of a thin film on the surface (s) and by the distribution of impurities in the thin film as well as the underlying substrate. In some instances the thin film and the impurities present therein are deliberately added while in other cases both are accidental. For deliberate additions the concentrational and spatial profile of the film and its impurities must satisfy certain criteria to produce the property specifications required (Hannay, 1960). Malm (1969) has characterized surface films or thin films into the categories of: alien surface contamination, intentional surface contamination, and sputtered films. This characterization is based on the distinctions implied by the titles used and, in fact, the distinction between the latter two categories is based on 401

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possible differences in the mode of deposition. Although each of these types of films present unique analysis problems, it is possible to generalize the analysis requirements to a degree.

13.2 GENERAL STATEMENT OF THE ANALYSIS PROBLEM Regardless of the type of surface film encountered, the analysis problem typically assumes several aspects. In some cases, the only requirement imposed is to establish the identity of the principal components of the film. Assuming that the analysis technique is capable of spatially differentiating between the film and the substrate, this identification is actually analogous to determining the composition of a major constituent, i.e., the concentration level of that constituent is greater than a few percent of the total composition of the film. From the standpoint of analytical sensitivity, this type of analysis does not generally present a major problem except for very thin films. In the more stringent case it is usually necessary to determine the variation in the composition of the film in the X-, F-, and Z-directions and also the variations in its Z-axis dimension of thickness. A sufficiently large number of measurements at different points on the XY surface provides a pattern of the distribution of main components of the film in those dimensions. This requires, however, that one of two possible assumptions must be acceptable. The XY pattern will be valid if it can be assumed that the composition does not change significantly within the depth of sampling in the Z-direction. If this is true, then controlling the depth of sampling is not particularly critical. In the converse, if the composition changes with respect to the depth in either a uniform or nonuniform fashion, a valid XY composition pattern can be obtained only if the depth of sampling at each point can be controlled within reasonable limits. Up to this point, the film has been considered only on the basis of major component compositions. In many instances, however, it is necessary to determine the concentrations of the minor and trace elements contained in the film (Chupakhin et al.y 1969a, b). Again, determining the spatial distribution of these minor concentrations is of interest. This presents a twofold problem. In the first case, if the film is less than 5-10 /xm thick, careful control of sampling in the Z-direction is required if the thin film impurities are to be distinguished from the impurities in the substrate. Second, because the film may be as thin as a fractional monolayer, an analysis technique of high sensitivity may be a prime requisite. For example,

13

SURFACE AND THIN FILMS ANALYSIS

403

if a sample of l-μΐη depth is to be analyzed and the XY sampling area is 1 mm2, this represents a sampling volume of 10~6 cm3. At a density of 5 g/cm 3 , a total of 5 X 10~6 g of the thin film is sampled and, at an impurity concentration level of 1 ppm, a technique capable of detecting 5 X 10~12 g of the impurity would be required for the analysis. In fact, very few techniques are capable of providing the degree of spatial resolution coupled with the high sensitivity required for this type of analysis. To summarize, then, an analysis technique that is capable of directly sampling a surface with spatial resolution in the Z-direction of a fractional monolayer or a fractional micron at least is required. Generally, the spatial resolution in the XY plane may not be as critical although examples will be subsequently discussed wherein resolution in the micron range is desireable to permit the detection and identification of segregates in a three dimensional profile. In most instances, the second primary requirement of the analysis technique utilized is high sensitivity, i.e., the method should generally be capable of detecting at least 10~9 g of an element. The distribution of the major and impurity components of a thin film on the XY surface can be established by means of the electron microprobe which makes it possible to deal with a microvolume as small as 1 μΐη3 or less (Birks and Seebold, 1961). While the electron probe method offers a generally high absolute sensitivity for many elements, the small sampling volume nevertheless confines its applicability to concentration levels above approximately 0.1% by weight. The method is consequently not applicable to the examination of high purity materials for trace constituents on either a thin film or bulk analysis basis. Solids mass spectrometry, on the other hand, offers the high absolute sensitivity required coupled with the general capability of determining all elements either on a simultaneous or single element basis. Consequently, considerable effort has been devoted to the application of mass spectrometry to the solution of the types of problems outlined above. The development of the mass spec trome trie applications has generally taken three routes. Efforts have been devoted to adapting the units utilizing a spark source, and which do not offer particularly high spatial resolution, to these analysis problems. In other instances, attempts have been made to replace the spark source on existing spectrometers with a vaporizing and ionizing medium offering higher spatial resolution in the Z-direction at least, e.g., laser ion sources. And, finally, new instruments such as the ion microprobe have been developed which are intended to offer an optimum combination of the spatial resolution requirements of the vaporization-ionization system and the sensitivity-selectivity requirements of the analyzer-detector system. The developments, capabilities, and limitations of each of these areas of investigation will be examined in

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the following sections utilizing actual applications as the mode of comparison. 13.3 13,3.A

INSTRUMENTATION AND APPLICATIONS Spark-Source Mass Spectrometry

The instrumentation associated with spark source units has been adequately described in previous chapters. Thus, only the modifications required for surface analysis will be presented herein. For films less than 5 μτη thick, the rf spark will normally penetrate entirely through the surface film and the spectrum observed will include elements from both the film and the substrate. The problem, then, is one of distinguishing the bulk substrate impurities from the surface material (Malm, 1969). Figure 13.1 illustrates how this distinction can be made. The data was acquired by collecting a series of equivalent exposures from the same point on the surface of an alloy (Berkey and Morrison, 1969). Two electrodes of an iron-nickel meteorite were sparked in opposition to

2

3 4 EXPOSURE NUMBER

5

Fig. 13.1 Differentiation between surface contamination and bulk impurities in the substrate with an rf spark (from Berkey and Morrison, 1969).

13

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SURFACE AND THIN FILMS ANALYSIS

COUNTER PROBE

SAMPLE ( V s U - ^ \

. _ / \ v

|

SPECTROMETER S U T

^-^

ION PATH

DIRECTION OF SAMPLE SCAN

F i g . 13.2 Electrode configuration for counterprobe analysis (from Berkey and Morrison, 1969).

each other at the surface of interest and the relative concentrations of several impurities were determined from each successive exposure by ion intensity measurements. The fact that nickel, a minor constituent in the meteorite, remains relatively constant in comparison to sulfur and copper verifies that the latter elements were present as surface contaminants. Moreover, it appears that the depth of the sulfur contamination is considerably greater than that for copper. Notice also that the copper concentration reached a constant level coincident with the bulk concentration in the alloy. Although this data is qualitative with respect to the fact that no attempt was made to measure the depth of the spark crater for an exposure, it does indicate that differentiation between bulk and surface impurities is possible. It should also be noted that in this example, the alternating nature of the spark provides for sampling the surfaces of the two self-electrodes used and the relative concentration values are thus representative of an average for the two exposed surfaces. While the self-electrode technique is useful, it is often desireable to examine the surface of interest in a more definitive manner. Some increase in spatial definition with respect to both depth and area of sampling can be realized through the use of a probe electrode composed of a high purity material which is foreign to the sample and its constituents. A foreign probe arrangement that has been used (Berkey and Morrison, 1969) is shown in Fig. 13.2. The sample is actually tilted at an angle of approximately 45° to the plane of the spectrometer slit. The sharply pointed probe, however, is parallel to the slit plane and consequently forms an angle of 45° with the sample surface of interest. By using this arrangement the slit can be reasonably well illuminated with the ion beam produced. A series of analyses can be run at any location on the sample to

406

R. K. SKOGERBOE

COUNTER PROBE

TRANSLATION

Fig. 13.3 Translational-rotational scanning system for thin (adapted from Clegg et al., 1970).

films

analysis

obtain a depth profile. In addition, the sample can be moved in the direction indicated while holding the probe position fixed to determine the X-direction distribution and the sample can be rotated to the other faces to determine the impurity levels in the substrate material away from the thin film surface. In general, when point-to-plane sparking techniques such as this are used, the depth of the crater produced for a nanocoulomb exposure is of the order of 10 Aim (Chupakhin and Romendik, 1969). Some decrease in the depth of the sample crater can be obtained by sacrificing spatial resolution in the XY direction. This can be accomplished by increasing the diameter of the probe used to insure sampling over a larger area. Another method for decreasing the sampling depth is based on translation or rotation of the surface as developed by Kessler et al. (1967) and adopted by Clegg et al. (1970). The spinning-electrode system serves well as a means of minimizing the production of secondary products and it brings fresh sample to the spark at a rate compatible with the frequency of rotation so that only original material is ionized. It has been estimated that the volume of material vaporized by a single spark is in the 10~12 to 10~14 cm3 range (Kessler et al., 1967). Because the time associated with the ionization of a single increment of sample is less than a microsecond and because original material is always presented to the spark, the processes of degradation, recombination, and polymerization of organic ions which are normally observed in spark source work are prominently reduced. The method is therefore useful for identifying organic surface contaminants.

13

SURFACE AND THIN FILMS ANALYSIS

407

Using a fixed counter electrode and a rotary scan system, Hickam and Sweeney (1966) have reported the generation of individual spark craters 0.2 μπι and less in depth. At such small volumes the concentrational sensitivity was naturally quite poor. Regardless of the geometry of the individual spark craters, it is intuitively certain that the uniform sampling of a finite volume of material can be realized only by controlled sparking of the sample surface while scanning in two dimensions. The device of Clegg et al. (1970) depicted in Fig. 13.3 represents a unique and potentially very useful evolution of the spinning electrode system originally developed by Kessler et al. (1967) and Hickam and Sweeney (1966). Mechanistically, the sample is simultaneously rotated and traversed so a spiral path is traced out by the rf spark. Using selfelectrodes of silicon, Clegg et al. (1970) determined that 10 mg of silicon was consumed per hour with a pulse repetition rate (PPS) of 300/sec and a pulse duration (PD) of lOOMsec. Consequently, 10~8 g of material was vaporized per pulse and, assuming hemispherical craters, the radius of each would be 10 Mm. If the peripheries of the craters are not to overlap, the sample surface scanning speed would have to be 20 /zm/pulse or 6 mm/sec

5 10 AVERAGE PENETRATION DEPTH (MX I0"6)

Fig. 13.4 Scanning analysis of a boron doped silicon layer (9.8 X 10 * M) showing chloride concentration variations with depth (adapted from Clegg et al., 1970).

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R. K. SKOGERBOE

at 300 pulses/sec. If the analysis is initiated at the outer periphery of the sample at rotation-translation rates consistent with the "no overlap" criterion, and if these rates are maintained constant, crater overlap would increase as the radius of sample rotation decreases due to the translational movement. In fact, as this happens each successive crater produced should be slightly greater in depth as determined by the degree of overlap between successive sparks. Figure 13.4 presents an analysis example based on this type of measurement indicating that a significant amount of chlorine was introduced into the epitaxial silicon layer during growth. Clegg and associates (1970), indicate that the thickness of the layer to be removed can be set simply by proper choice of the spark duty cycle and report that a distinct layer of 10 μΐη thickness can be analyzed with a sensitivity of 100 ppma without interference from the underlying substrate. Another means of solving the problem of determining thin film composition is based on the removal of the film by selective dissolution which essentially sacrifices the XF-direction spatial resolution (Malm, 1969; Ahearn, 1961). The dissolved material is subsequently deposited on the surface of a pure electrode material, the solution is evaporated off, and the residue is analyzed by conventional mass spectrometric methods. On the basis of the current literature reports, spark source mass spectrometry can be used for many surface analysis problems and it appears that it is possible to detect surface contamination equivalent to 0.01 monolayer (Ahearn, 1961). While the above discussion has not been comprehensive, it is generally indicative of the present methodology and capability for the rf spark. Certainly, this type of instrumentation and these sampling techniques will continue to be used for surface analysis problems. 13.3.B

Laser Methods

Because the coherent light beams obtainable from lasers can be focused onto very small areas creating high radiation densities and concomitantly high surface temperatures, this approach has received a fair amount of attention as a means for vaporizing and ionizing materials for mass spectrometric analysis. A schematic diagram of such a system is given in Fig. 13.5 as it was incorporated into a double-focusing instrument by Honig (1964). The laser beam enters the window W, and is focused on the target material T by the lens L. As long as the polarity applied to the slit housing is positive, only positively charged thermal ions enter the spectrometer. On reversal of the polarity, electrons released at the target initiate a low voltage arc discharge which generates large ion currents and the vaporized material in this discharge is ionized by electron impact.

13

409

SURFACE AND THIN FILMS ANALYSIS

LASER BEAM

LFig. 13.5 Schematic of a laser ion source (adapted from Honig, 1964).

LOW VOLTAGE SUPPLY

The pulsed laser used by Honig (1964) operated with an energy output of approximately 1 J and produced on the order of 1010 ions/pulse. An ion current of approximately 10~5 A was obtained and deleterious space charge effects were consequently observed. Moreover, the large energy output produced large craters which would preclude the use of this particular system for surface or thin film studies unless the laser beam were defocused at the sample surface. Mossotti and associates (1969) used a CO2 gas laser which can operate continuously at room temperature with a power output of 100 W or less as a vaporization-ionization medium. The laser beam was focused onto a sample area having an effective diameter of 200-500 μΐη so power densities at the surface were as high as 106 W/cm 2 . Qualitatively, the laser system produced a simple spectrogram of singly charged ions which were primarily the principle components of the matrix. The ion currents generated were quite low and the concentrational sensitivity was not particularly impressive. This problem was alleviated by using an auxiliary rf spark discharge in the vapor plume. Although no attempt was made to do surface or thin film analysis, it may be concluded that this system would offer a reasonable approach to the problem. The use of a laser offers the prime advantage that the sample material does not have to be conducting. A pulsed He-Ne laser has been used by Board and Townsend (1966) to analyze thin films of gold and aluminum on various substrates using a peak power of 70 W, a repetition rate of 500/sec and a pulse duration of 2 X 10~7 sec; penetration depths of 100-1000 A were obtained with a

410

R. K. SKOGERBOE

crater diameter of 10 μτη. The identity of the substrate did not appear to produce significant differences in the crater dimensions or the analytical quantitation of the results. Although lasers have not been widely used to date, they do offer some unique possibilities for surface analyses. Perhaps paramount among these would be the ability to obtain controlled spatial resolution on nonconducting as well as conducting samples. Surely as the laser develops, its application to this type of problem will develop also. 13.3.C

Ion Bombardment Instruments

Some of the most interesting instruments developed for analysis of surfaces and thin films (among other types of samples) are those which use a primary ion beam to sputter material from the surface and subsequently analyze the sputtered vapor. The proper design of such an instrument provides spatial resolution at the sample that is comparable to that realized with the electron microprobe while maintaining the sensitivity characteristic of spark source techniques. Instruments of this type are commercially available which differ in terms of design and capability and will be discussed in their approximate order of development. The ion microscope (a sophisticated mass spectrometer) was developed by Castaing and Slodzian (Castaing and Slodzian, 1962; Castaing, 1964; Slodzian, 1964). This unit was designed for analysis of minor and major constituents as opposed to trace analysis. A schematic adapted from that by Slodzian (1964) is given in Fig. 13.6. The unit combines a sputtering source using rare gas ions, a spectrometer, and an image converter to form an ion microscope. In bombarding the area to be investigated an ion beam current of approximately 10 μΑ is used and the ions are accelerated to the 10-keV energy level. The secondary ions produced are mass analyzed and refocused onto an exit slit at the electronic imager. Those ions passing this slit may be mass selected and are accelerated to strike the cathode of the image converter to produce tertiary electrons. The latter are accelerated in the opposite direction to strike a fluorescent screen which images the spatial distribution of the selected component of the sample surface. It is reported in the references above that the resolution of the image converter is 5 μΐη with the possibility of improving this to 0.3 μπι through the use of higher magnification. In a study of an 8-μΐη film of copper it was determined that the copper could be removed at the rate of 1 μΐη/min (Slodzian, 1964). Thus the spatial resolution attainable with this instrument coupled with its sensitivity and its visual imaging capabilities offer unique possibilities for analysis.

13

411

SURFACE AND THIN FILMS ANALYSIS

ACCELERATING PLATE FOCUSING SAMPLE

ELECTRONICIMAGER

Fig. 13.6 Schematic of an ion microscope with sputtering source, trometer, and image converter (adapted from Slodzian, 1964).

mass

spec-

While the application of the ion microscope to problems of interest in thin films technology has been limited, the reports made to date (Castaing and Slodzian, 1962; Castaing, 1964; Slodzian, 1964; Joyes and Castaing, 1966; Castaing and Hennequin, 1966; Slodzian and Hennequin, 1966; Hennequin, 1967) allow one to predict that it will play an important role in future analysis of thin films. The ion microprobe mass spectrometer is another excellent instrument for the analysis of thin films as well as segregates within a bulk material. The unit made by GCA Corporation of Bedford, Massachusetts, for example, also uses 10-keV inert gas atoms to vaporize surface atoms and/or monolayers from the material of interest so that successive layers may be analyzed. The duoplasmatron ion source used in this instrument to produce secondary ions from spatially resolved areas of the sample has been described by Herzog and associates (1967) and Barrington et al. (1966).

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SKOGERBOE

Fig. 13.7 Schematic of the GCA ion microprobe mass spectrometer. Reprinted from Evans and Pemsler, Analytical Chemistry, 42, 1060 (1970). Copyright 1970 by the American Chemical Society. Reprinted by permission of the copyright owner.

Using this type of source, sputtering rates of 80 μΐη/hr have been observed over a 0.25 mm2 area by Capellen et al. (1965). These authors report that the surface removal rate per unit area may be lowered by enlarging the bombarded area. It is reported that the ion beam of the GCA unit can be focused to an area of 0.001 cm2 or defocused up to 0.5 cm2 (Capellen et al., 1965). Sputter rates between 10~2 and 10~4 cm/hr were observed depending on the area bombarded. Hence 10-500 Mg of material are consumed per hour and a capability of detecting 10~12 g of an element is required if a 1-ppm analysis is to be realized. A schematic of the double focusing GCA ion microprobe unit is shown in Fig. 13.7. The primary argon ions are accelerated toward the sample at up to 15 keV. The beam diameter and position at the sample are determined by the Einzel lens and the deflection plates. Beyond this point, the ion optics and readout systems are essentially standard. In applying the ion probe to thin films analysis an accurate knowledge of penetration depths and sputtering rates is essential. Since these parameters depend on the reproducibility and constancy of the primary ion beam, provision must be made for directly measuring the flux of bombarding ions. Evans and Pemsler (1970) used a Faraday cage to adjust the primary ion current and monitor it for constancy. Using this means of adjustment, they

13

413

SURFACE AND THIN FILMS ANALYSIS

were able to reproduce the features and character of spatial distribution at the sample to 3 % of the actual depth. One other instrumental parameter may have a significant effect on the usage of the ion bombardment system for thin films analysis (Evans and Pemsler, 1970). Atomic and molecular ions sputtered from the sample exhibit different distributions with respect to initial kinetic energy (Herzog et al., 1967). By examining the secondary ion yield as a function of the kinetic energy of the secondary ions, it was found that the number of molecular ions drops rapidly with increasing kinetic energy. The yield of atomic ions, however, decreases only slowly with increasing energy. Evans and Pemsler (1970) took advantage of this to greatly reduce the spectral interference between 180+ and (Ή 2 1 6 0) + , i.e., the accelerating potential and the electrostatic analyzer voltage were preset to accept only those ions of high initial kinetic energy ( 1 8 0 + ). Data presented by Evans and Pemsler (1970) aptly illustrates the general type of thin film analysis that can be accomplished with the ion probe system. When Tantalum is anodized first in H 2 16 0 and later in H 2 18 0, or D 2 18 0, a double film of Ta 2 16 0 5 and Ta 2 18 0 5 results. Because the voltage/thickness relation during anodization is well established (Yound, 1961), the thicknesses of the oxide layers were precisely known before the mass spectrometric analysis. Samples were sputtered in a homogeneous ion beam at a rate of 0.65 A/sec and the 180+ and 1 6 0 + intensities recorded as a function of depth. Figure 13.8 summarizes the results for 1 8 0 + which agree 1001 90

* *v

\X

o 965 A OF Τα 2 ,8 θ5\ x 2180 A OF Τα 2 , 8 0 5 *\

\ 1

70 [

"

»

3Λ

«te\

80

1

60 50

40 Γ 30 [■ 20 I 10

\

1 1 1 1, 1 , , ■

500

* \

\ ■

'

■ ™- ■

■

'

■

1000 1500 DEPTH (A)

■

■

■

'

■

2000

■

'

\ -^'

' ■

2500

Fig. 13.8 Percentage 180+ versus depth for Ta 2 0 6 anodized in H1602 followed by D21802- Reprinted from Evans and Pemsler, Analytical Chemistry, 42, 1060 (1970). Copyright 1970 by the American Chemical Society. Reprinted by permission of the copyright owner.

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RESOLVING SLIT — /

Y*

^

PRIMARY MAGNET EXTRACTOR

^ C / > *

OBJECT APERTURE (DUOPLASMATRON)

CONDENSER LENS SPHERICAL ELECTRIC SECTOR

<*>-

-CURRENT MONITOR

RETROFOCAL LENS -

■ BEAM SWEEP

BEAM SWEEP

OBJECTIVE LENS

PICKUP ELECTRODEDEFLECTOR ELECTRODE

SAMPLE

Fig. 13.9 ARL ion microprobe mass spectrometer (from Robinson et al., 1968).

well with the electrochemical thickness measurement. Other data presented indicate that a depth resolution of 20 A is possible for the GCA unit used by Evans and Pemsler (1970). An instrument similar to the GCA unit but patterned after that described by Liebl (1967) and having a number of unique design features is being marketed by Applied Research Laboratories. A schematic illustration is presented in Fig. 13.9 (Robinson et al., 1968). Primary ions produced in a hollow cathode duoplasmatron are accelerated up to 22.5 kV and mass analyzed in a wedge-shaped magnetic field to achieve two-directional focusing. The emerging ion beam is subsequently focused on the sample by a two-stage electrostatic lens system quite similar to that found in an electron probe unit. Secondary ions from the sample are turned through a 45° angle by the deflector electrode into the double-focusing mass spectrometer. The analyzer section of this instrument offers four fairly unique features. The double focusing mass spectrometer is operated without an entrance slit to increase ion transmission and is arranged to image the bombarded sample area directly on the resolving slit. Increasing throughput in this manner is feasible only when the bombarded area is small as it is in the ion probe. The electrostatic and magnetic sectors are operated to produce zero velocity dispersion and, consequently, ions of considerably different energies are well focused at the exit slit. Sputtered ions typically have a wide

13

415

SURFACE AND THIN FILMS ANALYSIS

range of energies and, without this feature, it would be necessary to use a finite increment of initial energies to obtain reasonable resolution with a concomitant loss in sensitivity. A further sensitivity gain is realized by using two-dimensional rather than cylindrical focusing so that a point at the object is imaged into a point at the exit slit. Finally, the retrofocal lens produces a virtual image of the bombarded area some distance behind its actual position. This reduces the angular divergence of the secondary ions, provides a convenient focusing adjustment, and increases the overall sensitivity by a factor of 4 (Robinson et al., 1968). The ion source used in the ARL instrument is shown in Fig. 13.10. It is similar to that described by Herzog et al. (1967a) except that the electrons used to sustain the discharge are obtained from a hollow cathode rather than a thermal filament. The hollow cathode modification is as stable as the thermal filament, has a much longer service life, and is capable of producing ions from solids deposited in the cathode as well as from gases. Total ion currents ranging from 1 to 20 μΑ can be produced, and resolved ion currents of several tenths of a microampere can be delivered to the sample. In essence, the current density at the sample surface ranges from 10 to 100 mA/cm 2 depending on operational conditions and the identity of the sputtering ions used. GAS INLET

HOLLOW CATHODE-

MMF

ION EXTRACTOR-

Fig. 13.10 Modified duoplasmatron ion source (from Robinson et al., 1968).

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R. K. SKOGERBOE

OKV ( + 30 KV)

Fig. 13.11 Schematic of an ion detector assembly (from Robinson et al., 1968).

The inclusion of a mass analyzer in the primary ion beam portion of the instrument is characteristic of only the ARL unit. Among the advantages which accrue from this feature are : a specific bombarding ion can be selected to obtain a higher degree of control on the chemical and physical properties of that species; it is possible to distinguish the bombarding ion from that sputtered when it is desired that the two be chemically identical by simply using one isotope for bombardment and measuring the sputtered yield of the other; bombardment with fragments or radicals, doubly charged species, or single atoms of diatomic gases (e.g., 0 2 ) is possible; and a primary beam of either positive or negative ions can be used (Robinson et al., 1968). The latter capability appears to be important in at least two general cases. The primary reason for choosing negative ions for bombardment is that it permits obtaining a high ion current which is stable in time without prior surface treatment when bombarding insulators. Second, electronegative gases such as the halogens typically have a high yield of negatives from the source and a dramatic increase in the beam flux can be obtained through the use of the more naturally occurring species. Of course, the use of

13

417

SURFACE AND THIN FILMS ANALYSIS

negatives also permits easy discrimination between the sputtered and the bombarding ions. The ion detector is a modification of that originally described by Berhard et al. (1961) and is shown in schematic form in Fig. 13.11. The choice of the detector was primarily dictated by the need to detect both positive and negative ions. The ion beam is shielded from the bias voltage which accelerates secondary electrons formed by impingement on the target to the phosphor. The light produced in the phosphor is sensed and amplified by the photomultiplier tube and this signal is subsequently amplified by an electronics system which is inherently simpler than that normally used in conjunction with an electron multiplier. In addition, the phosphor can be biased negative with respect to the ion target to suppress secondary electron emission and permit measurement of the ion current by connecting an electrometer to the target. Prior to the resolving slit two pairs of electrostatic plates may be used to deflect an ion beam along the optic axis. Thus, two ion beams may be alternately presented to the detector at a rate of 1 kHz. By applying a 50-/isec blanking pulse during the actual beam switching, two ion beams may be integrated in a two channel readout system without contribution from the opposing beam or ion beams which may lie between the two of interest. This arrangement permits the integration of the ion currents from any two ions having masses that do not differ by more than 7% and serves as a convenient and precise means for measuring isotopic ratios. Having described the general features of ion sputtering instruments, it is appropriate to consider the process of ion production in said units. Reviews which cover the production of sputtered atoms have been published by Kaminsky (1965) and Behrisch (1964). Obviously the sputtered atom yield is determined by the nature of the sample, the identity of the bomTABLE 13.1 SPUTTERED

ATOM

YIELDS

AT

50

KEV

BOMBARDING ION ENERGY"

Bombarding ion

Number of sputtered atoms per ion

Ne

4.5 10 26 42

Ar Kr Xe * Roi et al. (1966).

418

R. K. SKOGERBOE

1

4

3 >\-

Ar +

"1

rv

32 0 +

r1 \ I

CO

1

uj

1

+

I

z

40

IK

- Ί 1

V

\

50

loo

TIME (SEC)

so

ioo

Fig. 13.12 Variation of the sputtered Al ion yield with an inert gas, argon and an electronegative gas, oxygen (from Robinson et al.y 1968 and Andersen, 1969).

barding ion, the flux of the bombarding entity, and its energy. Roi et al. (1966) have shown that the sputtered particle yield increases with the bombardment mass and with the energy. The increased yield obtained by energy increase, however, reaches a plateau and declines thereafter (Kaminsky, 1965; Andersen, 1969). The sputtered atom yield per bombarding ion observed by Roi et al. (1966) at the 50-keV energy level is presented in Table 13.1. The results agree with the collision model calculations published earlier by Rol and associates (1960). Only the first collisions of the bombarding ion were considered important in this model and the yield is predicted to be directly proportional to the maximum possible energy transferred in that first collision and inversely proportional to the mean free path for collision. Andersen (1969) and Andersen and associates (1969) have studied the sputtered yields of many pure elements. Figure 13.12 illustrates the differences observed when pure aluminum is bombarded with an inert gas and an electronegative gas under conditions that are otherwise the same. The exponential fall of ion production with time on bombardment with an inert gas is attributed to surface chemistry. The capability of removing positive ions diminishes as the strongly bonded compounds on the surface resulting from chemisorption of reactive gases, e.g., oxides, are removed by the bombarding beam. When oxygen is used, the surface chemistry is controlled through the selection of the bombarding species, i.e., the oxide layer is continuously reconstituted rather than destroyed. The final level of

13

419

SURFACE AND THIN FILMS ANALYSIS

A1+ production realized with argon bombardment is probably representative of a dynamic equilibrium between the arrival rates of the bombarding ions and reactive gas molecules from the ambient gas in the instrument. This has been experimentally verified by Andersen (1969) and similar effects were observed for all metal surfaces. The role of chemisorbed gas layers in ion production was also studied for stainless steel (Andersen, 1969) and the results are depicted in Fig. 13.13. Again the intensity loss with time is observed for argon and one can observe the relative ability of these three elements to compete for the reactive gases drawn from the ambient environment—chromium being the strongest competitor. When oxygen bombards the material, the delay in the chromium intensity drop (compared to that for aluminum in Fig. 13.12) suggests that a larger layer of chromium oxide exists on the surface of stainless steel. One could also suggest that the amount of iron or nickel oxide on the surface is quite minimal. To support this mechanistic proposal, Andersen (1969) ran other experiments, one of which is summarized in

50 TIME (SEC)

100

Fig. 13.13 Sputtered ion intensity of Cr, Fe, and Ni from 304 stainless steel under oxygen and argon bombardment (from Andersen, 1969).

R. K. SKOGERBOE

420

4

>

3

z

Id

Ό I

50 (a)

100 TIME (SEC)

50

100 (b)

Fig. 13.14 Variation of ion intensity with time for a sputter ion source: (a) pure aluminum bombarded with argon ions; (b) spot from (a) later bombarded with oxygen (from Andersen, 1969).

Fig. 13.14. These results demonstrate that the original peak intensity is correlated with the presence of chemisorbed gases. In this case the oxygen content of the first few monolayers of the aluminum was quickly removed by argon bombardment. When the clean spot was bombarded with 16 0 2 + ions while monitoring the 160+ output from the sample, the output builds up to a high stable level in a manner similar to that observed for the Al ion output (Fig. 13.12). While these experiments were directed at gaining insight into the sputtered ion production process, they distinctly indicate the potential of ion sputtering mass spectrometry for surface and thin films analysis. Moreover, the fact that a variety of both positive and negative bombarding ions can be used suggests that such instruments might be useful in elucidating the surface chemistry of such systems. It is unfortunate that the actual application of the ARL ion microprobe to surface and thin films problems to date all fall into the privileged information category (Robinson, 1970). Thus one can only speculate on the power of this type of unit for thin films analysis. It is quite certain that applications to this type of problem will be published as more of these instruments find their way into the hands of the university and the industrial communities. Regardless of the type of instrumentation used for thin films analysis, quantitation of the results—particularly with regard to depth—will continue to be a primary problem. The solution will eventually be obtained through developments made in analyzing well characterized thin films

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systems. The various aspects of the concentrational quantitation problem have been aptly discussed in Chapter 6 and will not be repeated here. The summary presented herein is certainly not all-inclusive but is intended to serve as a general indicator of the current capabilities of mass spectrometric methods for the analysis of surfaces and thin films. The spark source, the laser source, and the sputter ion source units all offer unique capabilities amenable to the solution of certain problems. These capabilities will be increased in each case by the development of further refinements in the instrumentation and in our understanding of the rather complex processes involved in the actual analysis. The ultimate sensitivity and the spatial resolution that can be realized with mass spectrometric techniques are sufficiently attractive in comparison to other instrumental techniques to suggest that development will be very rapid in this area of research in the next few years. REFERENCES

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